Plate Boundary Deformation Project

The Integrated Instrumentation Program for Broadband Observations of Plate
Boundary Deformation, commonly referred to as ``Mini-PBO'', is a joint project
of the BSL, the Department of Terrestrial Magnetism at Carnegie Institution of
Washington (CIW), the IGPP at UC San Diego (UCSD), and the U.S. Geological
Survey (USGS) at Menlo Park, Calif. It augments existing infrastructure in
central California to form an integrated pilot system of instrumentation for
the study of plate boundary deformation, with special emphasis on its relation
to earthquakes. This project is partially funded through the EAR NSF/IF program
with matching funds from the participating institutions and the Southern
California Integrated Geodetic Network (SCIGN).

Because the time scales for plate boundary deformation range over at least 8
orders of magnitude, from seconds to decades, no single technique is adequate.
We have proposed an integrated approach that makes use of three complementary
and mature geodetic technologies: continuous GPS, borehole tensor strainmeters,
and interferometric synthetic aperture radar (InSAR), to provide a broadband
characterization of surface deformation. In addition, ultrasensitive borehole
seismometers will monitor microearthquake activity related to subsurface
deformation.

The project has three components. One is focused on the San Francisco Bay area,
and augments existing instrumentation along the Hayward and San Andreas
faults. Two borehole sites were drilled during the summer of 2001, at Ohlone
Park (OHLN) in Hercules and on San Bruno mountain (SBRN) near Brisbane, to
complement 6 existing ones (Figure 7.1). These holes are equipped
with newly fabricated borehole tensor strainmeters and seismometers, and
downhole pore pressure and tilt sensors will be added in the near future. One
site in the Marin Headlands (MHDL) is scheduled to be drilled before the end
of September 2001, and two additional sites will be drilled by the end of 2001,
at the Knox/Miller Park (KNOX) near Richmond and on Ox Mountain (OXMT) near
Half Moon Bay. The first two instrumented sites are also awaiting installation
of electronics, GPS receivers, and Quanterra recording systems.

The GPS antennas will be mounted at the top of the borehole casings, wherever
possible, in an experimental approach to achieve stable compact monuments
(Figure 7.2). The GPS stations will complement 18 existing Bay Area
stations of the Bay Area Regional Deformation (BARD) continuous GPS network,
which includes more than 50 stations in northern California. The data from the
GPS, strainmeters, and seismometers will be acquired on Quanterra data loggers
and continuously telemetered over frame relay to U.C. Berkeley, while data from
other low frequency sensors will be telemetered using the GOES system to the
USGS. Sampling rates are 100 Hz for strainmeters and seismometers, 1 Hz
atmospheric, and 30 second GPS through the Quanterra data loggers, and 600
second for low frequency data (including strainmeters, for redundancy) over the
GOES system.

All data will be made available to the community through the Northern
California Earthquake Data Center (NCEDC). In preparation for this, the BSL
and USGS have worked out procedures to archive data from 139 sites of the USGS
ultra-low-frequency (UL) geophysical network, including data from
strainmeters, tiltmeters, creep meters, magnetometers, and water well levels.
These data are available in SEED format and set the stage for the archiving and
distribution of data from the Mini-PBO stations.

The second component of this project is to link the BARD network in central and
northern California to the SCIGN network in southern California. The
distribution of these sites allows measurement of both near-field deformation
from fault slip on the San Andreas and regional strain accumulation from
far-field stations. As of September 2001, seven new continuous GPS sites are
operational (6 constructed through Mini-PBO and one by USGS) and two more are
under construction. The sites are located in an array centered in the
Parkfield area and span about 25 km on either side of the San Andreas Fault.
Thus, the new array augments the considerable geophysical instrumentation
already deployed in the area and contributes to the deep borehole drilling on
the San Andreas fault (SAFOD) component of Earthscope. The data are currently
downloaded daily by SCIGN and archived by SOPAC. These sites will eventually be
upgraded to real-time streaming and analyzed in instantaneous positioning
mode.

The third component is InSAR, which supports skeleton operations of a 5-m
X-band SAR downlink facility in San Diego to collect and archive radar. In
addition, some funds are used to develop an online SAR database for WInSAR
users. The ERS-1/2 SAR data, which extend from 1992 until present, offer the
only means for monitoring plate boundary deformation at high spatial
resolution over all of western North America. This data set is largely
unexplored mainly because data distribution is restricted by ESA and working
with the phase information requires a significant investment of a researchers
time. Our objective is to improve access to these data for plate boundary
research within the strict guidelines set by ESA.

Figure 7.1:
Location of existing (yellow) and planned (blue)
Mini-PBO sites in the San Francisco Bay area. Shown also (red) are currently
operating strainmeter (circles) and BARD (triangles) stations in the area.

This year, the BSL has focused primarily on site selection for the integrated
network in the Bay Area, in cooperation with the USGS. Figure 7.1
shows the planned configuration. Two borehole sites were drilled during the
summer of 2001, at Ohlone Park (OHLN) in Hercules and on San Bruno mountain
(SBRN) near Brisbane. These holes are equipped with newly fabricated borehole
tensor strainmeters and seismometers, and downhole pore pressure and tilt
sensors will be added in the near future. One site in the Marin Headlands
(MHDL) is scheduled to be drilled before the end of September 2001, and two
additional sites will be drilled by the end of 2001, at the Knox/Miller Park
(KNOX) near Richmond and on Ox Mountain (OXMT) near Half Moon Bay. The first
two instrumented sites are also awaiting installation of power, telemetry,
electronics, GPS receivers, and Quanterra 330 recording systems.

Permitting of five additional sites for 2002 installations are in the
preliminary stages. These include: sites on U.S. Coast Guard property on Yerba
Buena Island in the San Francisco Bay (YERB) and near the former Hamilton Field
military base in Marin (HAML), a site at St. Marys College in the East Bay
(STMC), and two sites in the south Bay in the Rancho San Antonio Open Space
Preserve (RSAN) and Castle Rock State Park (CSTR). Site evaluation of geologic
properties, sky visibility, ease of permitting, and power and telemetry, have
been performed at most of the sites. We are continuing to evaluate the Yerba
Buena site where a location suitable for both GPS and strainmeters has proven
difficult to find.

After careful review and testing of several current generation GPS receivers,
we have decided to purchase Ashtech MicroZ receivers, which use about half the
power of the Ashtech Z-12 receivers currently used in the BARD network. We are
also designing an experimental GPS mount for the top of the borehole casings to
create a stable, compact monument (Figure 7.3). The antennas, using
standard SCIGN adapters and domes for protection, will be attached to the top
of the 6-inch metal casing, which will be mechanically isolated from the upper
few meters of the ground. The casing below this level will be cemented fully to
the surrounding rock. Although this design takes advantage of the deep
anchoring of the casing, we will need to assess in the future whether other
effects, such as daily or annual thermal expansion of the upper few meters of
the casing, limit the long-term stability of the monument.

This project is sponsored by the National Science Foundation
under the Major Research Instrumentation (MRI) program
with matching funds from the participating institutions
and the Southern California Earthquake Center (SCEC).

Under Barbara Romanowicz's general supervision, and with Mark Murray as head
guru, André Basset, Bill Karavas, John Friday, Dave Rapkin,
Doug Neuhauser, Tom McEvilly, Rich Clymer, and Roland Bürgmann contribute
to the development of the BSL component of the Mini-PBO
project. Mark Murray and Barbara Romanowicz
contributed to the preparation of this chapter.

Figure 7.2:
Design of the Mini-PBO borehole installation, showing the emplacement
of the instruments downhole and the GPS receiver on the top.

Figure 7.3:
Design of the Mini-PBO GPS antenna mount on top of casing.